The invention relates to measuring the excursion of hydraulic actuators using laser time-of-flight techniques. More particularly, it relates to linear hydraulic cylinders having laser light pulse emitters and laser light detectors reflecting off moving internal surfaces of the hydraulic cylinder.
With the increased computerization of traditionally mechanical and hydro-mechanical systems, the ability to accurately control and position a wide variety of agricultural and construction equipment has become possible. Using proportional control valves and hydraulic server motors, agricultural and construction vehicles have been designed that can compensate for a wide variety of environmental variables to much more accurately control the motion and positioning of their various arms and linkages.
To provide the ever-more-accurate positioning and control that microprocessors are capable of, one must know ever more accurately the exact position of the various linkages, arms and actuators that comprise these vehicles. Other methods of determining the location of mechanical components of these vehicles included such things as rotary potentiometers and resistors mounted at pivoting joints, linear variable differential transformers coupled to the outside of extendable devices such as hydraulic cylinders that extend and retract, and linear variable resistors.
These older methods of determining the location of various components of the vehicles are now seriously outmoded. In particular, many of these components include several moving parts that are exposed to the elements, and therefore are prone to be broken, bent or otherwise become un-calibrated.
What it need, therefore, is a more robust system of determining the position of vehicle components that reduces the risk of breakage, mis-calibration, and provides a greater positional accuracy than these previous devices. It is an objective of the present invention to provide such a system.
In accordance with a first embodiment of the invention, a fluid actuated cylindrical actuator is provided that includes a cylinder having first and second ends, an end cap fixed to the first end of the cylinder and having a rod opening, a piston disposed in the cylinder, a rod coupled to the piston and extending from inside the cylinder to outside the cylinder and passing through the rod opening, a first light guide extending from inside the cylinder to outside the cylinder and adapted to transmit at least a first beam of laser light at a first frequency from outside the cylinder to inside the cylinder and to bar the passage of the fluid, and second light guide extending from inside the cylinder to outside the cylinder and adapted to transmit the at least a first beam of laser light at the first frequency from inside the cylinder to outside the cylinder and to bar the passage of fluid. The first light guide may be disposed to transmit the first beam of laser light substantially along a longitudinal axis of the cylinder such that the first beam impinges on a reflective portion of the piston over substantially an entire range of piston travel. The second light guide may be disposed to receive the first beam after it has been reflected off the piston. The optical distance between the first light guide and the second light guide may be a function of the degree of extension of the rod outside the cylinder. The first beam of light may be reflected off a first surface inside the cylinder where the first surface is coupled to the rod and configured to move with the rod. The first beam of light may be reflected off a second surface fixed with respect to the cylinder and movable with respect to the rod and a third surface fixed with respect to the rod and movable with respect to the cylinder. The first beam may vary in optical length when the rod is moved with respect to the cylinder an amount equal to at least four times an axial distance the rod travels.
In accordance with a second embodiment of the invention, a hydraulic actuator for an agricultural or construction vehicle is provided, the actuator including a cylinder having a substantially circular internal diameter and a longitudinal cylindrical axis, a piston having a substantially circular outer diameter and configured to be received in and hydraulically sealed against the inner diameter of the cylinder, a piston rod with a substantially circular outer rod diameter that is fixed to the piston and extends from the piston inside the cylinder, through a first end wall of the cylinder to a location outside the cylinder, wherein the first end wall is disposed to enclose and seal a first end of the cylinder and is substantially perpendicular to the longitudinal axis of the cylinder, a second end wall fixed to the cylinder and disposed to seal a second end of the cylinder substantially perpendicular to the longitudinal axis of the cylinder, the second end wall including a first optical path configured to transmit a beam of laser light through the second end wall to a reflective surface fixed to the piston and further including a second optical path configured to transmit the reflective beam of laser light back through the end wall, a first optical fiber optically and mechanically coupled to the second end wall to transmit the beam of laser light from a remote laser light source to the first optical path and a second optical fiber optically and mechanically coupled to the second end wall to transmit the reflected beam of laser light to a remote laser light receiver.
The first and second optical paths may include at least one hermetically sealed fiber optical feed-through or connector extending through the second end wall. At least one fiber optic feed-through or connector includes an adjustable focal length lens disposed in the beam of laser light and configured to adjust a focal length of the beam of laser light within the cylinder. At least one fiber optic feed-through or connector may include a threaded end cap with the adjustable focal length lens fixed to the end cap. The hydraulic actuator may include another hermetically sealed fiber optic feed-through or connector extending through the second end wall. The first and second optical fibers may be multi-modal optical fibers. The actuator may also include a first laser diode configured to emit a beam of laser light at a wavelength in the range of 840 to 980 nanometers. The hydraulic actuator may also include a first photo-diode configured to receive the beam of reflective laser light and generate an electrical signal indicative of at least one characteristic of the beam. The actuator may also include a second laser diode configured to emit the beam of laser light in the range of 430 to 1300 nanometers. The actuator may include a second photo diode configured to receive a beam of laser light and generate an electrical signal indicative of at least one characteristic of the beam.
In accordance with a third embodiment of the invention, a method of determining the position of the piston of the actuator described in the previous paragraph includes the steps of generating the beam of laser light, reflecting the beam of laser light off a surface fixed to move axially with the piston, receiving the reflected beam of laser light, and calculating a time-of-flight. The step of generating the beam may include the step of generating the beam with a wavelength of between 430 and 1300 nanometers. The step of generating the beam may include the step of generating the beam with a wavelength in the range of 840 and 980 nanometers. The step of generating the beam may include the step of generating a sequence of individual pulses of light, and the step of calculating a time-of-flight may include the step of determining the time-of-flight of at least one pulse in the sequence of individual pulses of light.